1. Field of the Invention
The present invention relates to an electromagnetic brake and a drive force distributing device for a vehicle using the electromagnetic brake.
2. Description of the Related Art
A differential is located in a power train of a vehicle to maintain torque distribution between right and left wheels of the vehicle such that torque is equally divided between the right and left wheels and to rotate the outside wheel faster than the inside wheel in cornering, thereby reliably obtaining smooth cornering. While the primary role of the differential is to obtain smooth cornering as mentioned above, there is a case that one of the right and left wheels may be caught to slip in a muddy place during rough-road running. In this case, the resistance from the road to the wheel caught to slip in the muddy place is small, so that torque is almost transmitted to this slipping wheel and hardly transmitted to the other wheel. As a result, the drive force for driving the wheels becomes lacking to cause a problem that the slipping wheel cannot escape from the muddy place. This problem is a defect inherent to a general differential.
Known is a special type of differential having a differential motion limiting mechanism capable of compensating for the above inherent defect of a general differential. This type of differential is referred to as a limited slip differential (LSD). A planetary gear type differential is generally known in the art. For example, such a planetary gear type differential gear assembly having a limited slip differential mechanism composed of an electromagnetic clutch and a multiplate clutch is disclosed in Japanese Patent Laid-open No. Hei 6-33997.
In this differential gear assembly, an attraction force between a solenoid and an armature forming the electromagnetic clutch is applied to the multiplate clutch to press it and selectively control an engaging force generated in the multiplate clutch. A connecting member consisting of a plurality of bars is located between a pressure plate of the multiplate clutch and the armature. That is, one end of each bar of the connecting member is fixed to the pressure plate of the multiplate clutch, and the other end comes into abutment against an inner circumferential portion of the armature when the solenoid is operated.
In the conventional differential gear assembly mentioned above, the plural bars fixed to the pressure plate extend in a direction substantially perpendicular to the pressure plate. Accordingly, in the case that any of these bars are inclined to the pressure plate, there is a problem that a pressing force of the armature attracted by the solenoid to press the pressure plate of the multiplate clutch may not be uniformly transmitted to the pressure plate. Further, in the conventional differential gear assembly described in the above publication, the electromagnetic clutch controls the engaging force of the multiplate clutch, so that the plural bars as pressure members are located so as to correspond to the inner circumferential portion of the armature. However, in a multiplate brake structure having a plurality of brake plates and a plurality of brake discs, these brake plates and brake discs are generally located so as to correspond to an outer circumferential portion of the armature from the viewpoint of the structure. Accordingly, it is difficult that the conventional structure described in the above publication such that the multiplate clutch is operatively connected to the armature at its inner circumferential portion is applied to the multiplate brake structure without any changes.
The present applicant has already proposed an electromagnetic brake solving the above problem (Japanese Patent Application No. 2001-267785). This electromagnetic brake in the related art includes a multiplate brake mechanism, a ringlike core member having an annular exciting coil, and a ringlike armature member opposed to the annular exciting coil of the core member. The electromagnetic brake further includes a cylindrical pressure member provided so as to surround the outer circumferential surface of the core member and be movable in a direction of pressing the multiplate brake mechanism as being guided by the core member, the pressure member having one end fixed to an outer circumferential portion of the armature member and another end engaged with the multiplate brake mechanism.
In this electromagnetic brake, an air gap is defined between the armature member and the core member, and the armature member is attracted to the core member by passing a current through the exciting coil, thereby engaging the multiplate brake mechanism. The thrust by the exciting coil is largely influenced by the amount of the air gap. The cylindrical pressure member of this electromagnetic brake has a function of transmitting the thrust by the exciting coil to the multiplate brake mechanism and a function of radially positioning the armature member to allow circumferentially uniform displacement of the armature member.
In the conventional electromagnetic brake, priority is given to the accuracy of radial positioning of the armature member, so that the cylindrical pressure member and the core member must be formed of the same type of material in consideration of linear expansion due to a temperature change. Accordingly, the cylindrical pressure member is formed of stainless steel in the conventional electromagnetic brake. On the other hand, a housing for accommodating the electromagnetic brake is formed of aluminum alloy for the purpose of weight reduction, and the core member is fixed to the housing. As a result, the coefficient of linear expansion of the housing to which the core member is fixed does not match with that of the cylindrical pressure member interposed between the multiplate brake mechanism and the armature member. Accordingly, in the electromagnetic brake as a whole, the air gap between the armature member and the core member changes with a temperature change, causing variations in engaging force (braking force) of the multiplate brake mechanism.
Accordingly, in the case of applying this electromagnetic brake to a drive force distributing device for a vehicle, there arises a problem such that it is difficult to properly distribute a drive force in response to a temperature change. Further, in the conventional electromagnetic brake, the multiplate brake mechanism is arranged at a radially outer position, and the ringlike armature member is arranged at an axially outer position opposite to the multiplate brake mechanism with respect to the core member. This structure is intended mainly to make the armature member axially straight push the multiplate brake mechanism through the cylindrical pressure member, so that the ringlike core member and the ringlike armature member are large in outer diameter. Further, in order to suppress the effects of changes in the air gap between the core member and the armature member, inclination of the armature member, etc., machining accuracy is required, for example, radial positioning is required between the outer circumference of the core member and the inner circumference of the cylindrical pressure member.